As cache is in SYNC mode I would say that this is 'normal' behavior. The non-GC cache waits for acknowledgment from the frozen cache until it received it or times out (SyncReplTimeout set to 15secs in your case)
If you cannot tune the GC to avoid "Stop the world" scenarios, here are some suggestions from a cache perspective:
1) use ASYNC replication if business allows it. This way the non-GC cache won't wait for the frozen cache to ACK
2) reduce the SyncReplTimeout. If so the non-GC cache will throw an exception on operation, and give quicker feedback to the user.
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